The invention relates generally to photocopiers employing liquid toner developer and particularly to apparatus for stripping sheets from a photosensitive drum after transfer of the developed image from the drum to the sheet.
In a photocopier employing a rotating photosensitive drum surface, the drum surface is first electrically charged. It is then exposed to a light pattern generated by a scanner passing over an original and focused on the drum, to form a latent electrostatic image on the drum surface. The latent image is developed, and the developed image is transferred to a sheet of transfer material brought into contact with the surface image. The sheet is then transported away from the surface toward an exit tray. It becomes the desired copy of the original.
The sheet brought into contact with the surface of the drum ordinarily adheres closely to it, because of the electrostatic fields created during the photocopying process and, in the case of liquid toner copiers, because of the surface tension created by the liquid toner. The sheet must be stripped off the drum, and various methods have been developed to perform this function with a minimum of disturbance to the other elements of the copying system. One method is to blow a stream of air between the copy sheet and the surface of the drum to lift the leading edge of the copy sheet so that it may be gripped and conveyed from the drum. This method has the disadvantage of potentially disturbing the toner particles adhering to the copy sheet. In another method known to the art the sheet stripping is accomplished by claws brought into engagement with the drum. This method has the disadvantage of requiring the claws to be in contact with the drum surface, so the drum surface may become worn or damaged.
Another method, described more fully in U.S. Pat. No. 3,936,045, provides for stripping members in strip or string form in which the strip or string is pressed against an edge of the drum photosensitive surface. When the copy sheet is brought to the drum, an edge of the copy sheet slides along the member. The member includes a forward portion increasingly spaced away from the drum surface to guide a portion of the leading edge of the copy sheet to engagement by a strip-off device. A disadvantage of this approach is that an entire edge of the copy sheet is kept from contact with the drum surface and is therefore unavailable for copying. The pressure of the member against the drum photosensitive surface also may wear or damage the surface.
It is a principal object of this invention to provide a sheet stripping apparatus that minimizes interference with the photocopying process and reduces potential damage and wear to portions of the photosensitive surface. It is another important object of the invention to provide a sheet stripping apparatus that maximizes the amount of transfer sheet surface available for copying. A further object of the invention is to provide a sheet stripping apparatus that is reliable, durable, and substantially maintainance free.